This invention relates to digital-to-analog and more specifically to digital-to-analog drivers implementable in field effect transistor devices, the type which may be integrated in large scale semiconductor means, which drivers is capable of driving a speaker or other voice coil means.
Disclosed is a talking learning aid which utilizes several integrated circuits in the construction thereof. The integrated circuits are discussed in detail herein and include a speech synthesis chip, a controller chip and one or more Read-Only-Memory chips. The voice synthesizer chip includes a digital to analog drivers circuit which is capable of directly driving a small speaker or other voice coil means. Thus, this patent teaches not only the digital-to-analog drivers of this invention but also a preferred embodiment in which it is utilized. It will, of course, be appreciated by those skilled in the art that other embodiments may also be found for the disclosed digital-to-analog drivers circuit.
Preferably, when providing the aforementioned speech synthesis chip, the chip is implemented in standard field effect transistor large scale integration techniques, such as P-Channel MOS and preferably the digital-to-analog drivers is integratable on the same chip as the voice synthesizer. Preferably, to eliminate any need for an amplifier circuit, the digital-to-analog drivers circuit includes sufficient gain for directly driving a speaker or other voice coil means.
It was, therefore, one object of this invention that a digital-to-analog drivers be implemented in standard FET large scale integration techniques.
It was another object of this invention that the digital-to-analog drivers circuit be directly coupled to a speaker or other voice coil means, or at least, coupled to a speaker or other voice coil means without the need for additional amplification.
It was yet another object of this invention that the gain of the digital-to-analog driver be essentially insensitive to the threshold voltage of the semiconductor material.
The foregoing objects are achieved as is now described. A plurality of switching field effect semiconductive constant current sources, each of which has a control electrode and two current carrying electrodes, are connected in parallel at the current carrying electrodes thereof. Each constant current sources includes a field effect switching device and a field effect current source device, the gate of the switching device providing the aforementioned control electrode. The digital signal is applied to the gates of the switching devices and the width to length ratios of the active areas of these devices, as well as the current source devices, differ by a factor of two from each other. That is, a first one of the devices has a given width to length ratio while the width to length ratio of a second one of the devices is twice that of the first and the width to length ratio of a third one of the devices is twice that of the second and four times that of the first, and so forth. Preferably, one side of the sources is coupled to a first voltage potential. The other side, on the other hand, is connected via to switching transistors to the terminals of a speaker, other voice coil means or any other differential input device. The second voltage potential may be alternatively coupled via pair of switching transistors to the terminals of the voice coil or speaker or via a center tap associated with the speaker or voice coil means or other differential input device. Further, an embodiment using a center-tapped transformer is also disclosed. Preferably, the digital signal includes a sign bit and a plurality of magnitude bits, the plurality of magnitude bits being applied to the gates of the plurality of switching devices whereas the sign bit is applied in true and complemented form to the switching transistors.